In recent years, there have not been many cases in which high-frequency filters that are used in a microwave band and the like are made up of dielectric resonators using dielectric substances with high relative dielectric constants for the purpose of downsizing and enhancement in performance by downsizing. By forming a dielectric substance into a block in a specific size and shape, a dielectric resonator can be resonated at a desired frequency that is fixed by the size and shape of the dielectric substance and the relative dielectric constant.
A dielectric resonator using ceramics with a high relative dielectric constant (dielectric ceramics) as the material of the dielectric substance is widely known. When an electric field is applied to the molecules that configure the dielectric ceramics, the bound electrons in the molecules migrate and are polarized, and thereby the dielectric ceramics shows a high relative dielectric constant. As the relative dielectric constant of dielectric ceramics, the dielectric ceramics having relative dielectric constants of 20 to 100 can be generally put to practical use when smallness of loss at a high frequency and temperature stability are taken into consideration.
As a dielectric resonator, the dielectric resonator using an artificial dielectric substance (an artificial dielectric resonator) has been also proposed (For example, Patent Literature 1). An artificial dielectric substance is formed from assembly of a number of metallic pieces. The artificial dielectric substance behaves as a dielectric substance as a result that free electrons that are present in the metallic pieces migrate and are polarized when an electric field is applied thereto, and can obtain a high equivalent relative dielectric constant in accordance with the size and the shape of the metallic pieces, depending on the number of free electrons and the length of the migration distance. Note that artificial dielectric substances are disposed in a certain base material in order to retain the respective metallic pieces.
Further, an artificial dielectric substance has such anisotropy that the relative dielectric constant changes depending on which direction of the metallic pieces an electric field is applied as described in Patent Literature 1. Due to the anisotropy, the metallic pieces are disposed so that in resonance (a basic mode) at a desired frequency, the relative dielectric constant becomes high, and at other resonances (spurious mode) at frequencies relatively close to the desired frequency, the relative dielectric constant becomes low, whereby the artificial dielectric resonator can separate these frequencies, and thereby can suppress the spurious mode.